mark some positional-only arguments (#4693)

https://github.com/python/mypy/pull/9626 will make stubtest a little bit
stricter about positional-only arguments for dunders like __init__

Co-authored-by: hauntsaninja <>

stdlib/3/collections/__init__.pyi

@@ -74,7 +74,7 @@ else:
 
 class UserDict(MutableMapping[_KT, _VT]):
     data: Dict[_KT, _VT]
-    def __init__(self, dict: Optional[Mapping[_KT, _VT]] = ..., **kwargs: _VT) -> None: ...
+    def __init__(self, __dict: Optional[Mapping[_KT, _VT]] = ..., **kwargs: _VT) -> None: ...
     def __len__(self) -> int: ...
     def __getitem__(self, key: _KT) -> _VT: ...
     def __setitem__(self, key: _KT, item: _VT) -> None: ...